Nanoparticles of 1-100 nm have broad applications in biology, such as in labelling, imaging, drug delivery, separation and optical sensing. The size of nanoparticles plays an important role in determining their properties as well as their effectiveness in bioapplications. The synthesis of monodisperse nanoparticles is critical to the tailoring and optimization of size-dependent characteristics.
Nanoparticles also require suitable surface functional groups in order to conjugate with biomolecules. Most biomolecules have carboxylic acid, primary amine, alcohol, phosphate, or thiol groups, and nanoparticles that are functionalized with primary amines, carboxylic acids and thiol surface groups can be covalently conjugated with biomolecules via amide, disulfide and ester bonds.
Many methods are currently available for nanoparticle synthesis. Most synthetic approaches are based on organic solvent routes where particles are coated with hydrophobic/lipophilic organic stabilizer molecules. The nanoparticles as synthesized cannot be used directly for biofunctionalization/bioapplication because they are insoluble in water, they do not have required functional groups for bioconjugation, and/or they are unstable toward various processing steps for bioapplications.
Currently, several strategies are available to solve these problems, for example by exchanging the original stabilizer with surfactant/ligand molecules or polymers, silanization, and dendron bridging. The key issue is to deal with the sensitive surface chemistry of the nanoparticles, and the colloidal stability of the nanoparticles in aqueous phase. Ligand-exchanged nanoparticles are less stable and often lead to irreversible aggregates as the nanoparticles lose their organic shells. To solve this problem, ligand-exchange followed by cross-coupling is used to provide covalent bridging surrounding the particles. Silanization and dendron bridging are unique alternative schemes to generate covalent bridging of stabilizer shell surrounding the nanoparticle. Using this approach, stable quantum dot and metallic nanoparticle dispersions can be prepared. However, these approaches often produce water-insoluble nanoparticles, because of interparticle bridging.